|
A T-antenna, T-aerial, flat-top antenna, or top-hat antenna is a vertically polarised simple wire radio antenna used in the VLF, LF, MF and shortwave bands. T-antennas are widely used as transmitting antennas for amateur radio stations, long wave and medium wave broadcasting stations. They are also used as receiving antennas for shortwave listening. The antenna consists of one or more horizontal wires suspended between two supporting radio masts or buildings and insulated from them at the ends.〔〔 A vertical wire is connected to the center of the horizontal wires and hangs down close to the ground, connected to the transmitter or receiver. Combined, the two sections form a 'T' shape, hence the name. The transmitter power is applied, or the receiver is connected, between the bottom of the vertical wire and a ground connection. The T-antenna functions as a monopole antenna with capacitive top-loading; other antennas in this category include the inverted-L, umbrella, delta, and triatic antennas. It was invented during the first decades of radio, in the wireless telegraphy era, before 1920. ==How it works== When the length of the wire segments are shorter than a quarter wavelength (λ/4) of the radio waves, as is typical for use below 1 MHz, the antenna functions as a vertical electrically short monopole antenna with capacitive top-loading.〔 Because the two horizontal arms of the "T" have equal but oppositely-directed currents in them, which causes the radio waves from them to cancel far from the antenna, and because of similar cancelling ground currents, the horizontal wire radiates little radio power.〔( Rudge, 1983, p.554 )〕 Instead it serves to add capacitance to the top of the antenna.〔 This increases the currents in the upper portion of the vertical wire ''(see drawing at right)'', increasing the radiation resistance and thus its efficiency,〔 allowing it to radiate more power, or in a receiving antenna be more sensitive to incoming radio signals. The top load wire can increase radiated power by 2 to 4 times (3 to 6 dB) for a given base current.〔 However, the antenna is still typically not as efficient as a full-height λ/4 vertical monopole,〔 and has a higher Q and thus a narrower bandwidth. T antennas are typically used at low frequencies where it is not practical to build a quarter-wave vertical antenna because of its height,〔 and the vertical radiating wire is often very electrically short, only a small fraction of a wavelength long, 0.1λ or less. Since the radiation resistance and efficiency increases with height, the antenna should be suspended as high as possible. The antenna has a base impedance with a capacitive component that must be tuned out with an added inductor. To increase the top-load capacitance, several parallel horizontal wires are often used, connected together at the center where the vertical wire attaches.〔 The capacitance does not increase proportionally with the number of wires, however, because each wire's electric field is partially shielded from the ground by proximity to the adjacent wires.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「T-antenna」の詳細全文を読む スポンサード リンク
|